[css-shapes] Changed initial value of shape-image-threshold

Hey all,

The draft of CSS Shapes had an initial value of shape-image-threshold set
to 0.5, which would create a shape that enclosed all the pixels of the
specified image whose opacity was greater than 50%. It's a much more
common case to want to construct a shape that excludes just the
fully-transparent pixels, so I've changed the initial value to 0.0.

Thanks,

Alan